[ALPS-users] mps_optim: measurement - bug?

[Michele Dolfi]

Hi Mateusz,

I didn’t manage to follow up on this earlier.
Could you please send the model file that you are using?


Michele

--
ETH Zurich
Dr. Michele Dolfi
Institute for Theoretical Physics
HIT G 32.4
Wolfgang-Pauli-Str. 27
8093 Zurich
Switzerland

dolfim at phys.ethz.ch
www.itp.phys.ethz.ch

+41 44 633 78 56 phone
+41 44 633 11 15 fax 






> On Jan 15, 2017, at 12:28 PM, Michele Dolfi <dolfim at phys.ethz.ch> wrote:
> 
> Dear Mateusz,
> 
> Which version of the code are you using?
> We recently rewrote how measurements are parsed (specially for measurement expressions).
> 
> Could you please try with a version newer than r7807?
> 
> 
> Best,
> Michele
> 
> --
> ETH Zurich
> Dr. Michele Dolfi
> Institute for Theoretical Physics
> HIT G 32.4
> Wolfgang-Pauli-Str. 27
> 8093 Zurich
> Switzerland
> 
> dolfim at phys.ethz.ch <mailto:dolfim at phys.ethz.ch>
> www.itp.phys.ethz.ch <http://www.itp.phys.ethz.ch/>
> 
> +41 44 633 78 56 phone
> +41 44 633 11 15 fax 
> 
>> On 14 Jan 2017, at 18:17, Mateusz Łącki <mateusz.lacki at gmail.com <mailto:mateusz.lacki at gmail.com>> wrote:
>> 
>> Dear All.
>> I would like to report a likely bug.
>> 
>> I play with a simple transverse ising model for S=1/2. I  take a short chain of length say 8, compute magnetization in x and z direction. When I compute the same input with mps_optim and sparsediag i obtain different results. Both codes give me different magnetizatios in mps_optim measurement seems to be wrong by a factor.
>> 
>> I changed the definition of a default model  “spin” (see below), the coupling is along x axis and field is along z. I consider the following input:
>> 
>> LATTICE="inhomogeneous chain lattice"
>> MODEL_LIBRARY="models.xml"
>> 
>> MODEL="spin"
>> J=1
>> SWEEPS=8
>> chkp_each=8
>> 
>> MEASURE_LOCAL[Local magnetization Xa]=Splus
>> MEASURE_LOCAL[Local magnetization Xb]=Sminus
>> MEASURE_LOCAL[Local magnetization X1]=Sx
>> MEASURE_LOCAL[Local magnetization X2]=Sxx
>> MEASURE_LOCAL[Local magnetization Z]=Sz
>> MAXSTATES=40;
>> NUMBER_EIGENVALUES=1;
>> 
>> {h=0;Gamma=0;L=8} 
>> 
>> *******************
>> In the above the Sxx operator is defined exactly the same as Sx:
>> <SITEOPERATOR name="Sx" site="x">
>>  1/2*(Splus(x)+Sminus(x))
>> </SITEOPERATOR>
>> 
>> but with 1/4 factor, not 1/2:
>> <SITEOPERATOR name="Sxx" site="x">
>>  1/4*(Splus(x)+Sminus(x))
>> </SITEOPERATOR>
>> 
>> 
>> There are 2 runs that are important for my message:
>> 
>> RUN A):
>> running the above input with sparsediag gives (at any site):
>> Local magnetization Xa=0.5
>> Local magnetization Xb=0.5
>> Local magnetization X1=0.5
>> Local magnetization X2=0.25
>> 
>> Which makes sense, as Sx=0.5*(Jplus + Jminus)
>> 
>> RUN B):
>> running the above input with mps_optim gives (at any site):
>> Local magnetization Xa=0.5
>> Local magnetization Xb=0.5
>> Local magnetization X1=1.0
>> Local magnetization X2=1.0
>> 
>> 
>> My conclusion:
>> It seems that measurement ignores the factor 1/4 in the definition of Sxx and 1/2 in the definition of Sx. If it is indeed the case (not stupid mistake on my side), would it be possible to issue a patch?
>> 
>> If I change 
>>  <BONDTERM source="i" target="j">
>>    <PARAMETER name="J#" default="J"/>
>>    -J#*Sx(i)*Sx(j)*4
>>  </BONDTERM>
>> 
>> into 
>>  <BONDTERM source="i" target="j">
>>    <PARAMETER name="J#" default="J"/>
>>    -J#*Sxx(i)*Sxx(j)*4
>>  </BONDTERM>
>> 
>> then I get a correct factor 4 reduction in energy. So it seems only the mesurement ignores the numerical factor.
>> 
>> If I exchange x-z direction in the Hamiltonian everything seems fine.
>> 
>> Best,
>> Mateusz Łącki
>> 
>> 
>> 
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> 
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